A lithium ion rechargeable battery, which is a nonaqueous electrolyte rechargeable battery, is widely used in portable electronic devices, such as video cameras, portable audio players, mobile phones, and notebook computers, which have been becoming smaller, lighter, and more powerful. Such electronic devices, on one hand, have been requiring lithium ion rechargeable batteries of still higher capacity and longer life. On the other hand, the circumstances of use of these portable devices have recently been changing, which in turn changes the characteristics required of the battery. It is often observed recently that a battery is charged continuously to keep it fully charged (sometimes referred to as continuous charging hereinbelow). A battery in a notebook computer is often used while it is connected to an external power supply, which places the battery in continuous charging. The same is applied to mobile phones, which is often charged continuously.
Under such circumstances, the cathode material of a lithium ion rechargeable battery deteriorates faster than in normal use. Such deterioration is assumed to occur by a mechanism wherein the electrical current or voltage acts on a cathode material from which Li has been eliminated by charging, to cause further elimination of Li therefrom, resulting in decomposition of the cathode material. When the deterioration proceeds, metal ions generated by decomposition of the cathode material precipitate in the battery system to cause micro-short circuit, which may lead to heat generation and ignition. An external circuit may be provided to minimize the electric current and voltage acting on the cathode for inhibiting the deterioration. Yet, improvement of stability during continuous charging is an essential problem lying in a cathode material of a lithium ion rechargeable battery. Further, it is a recent trend to use a lithium ion rechargeable battery at a high voltage of 4.3 V or higher. Charging at a higher voltage makes available a larger capacity, while the amount of Li leaving the cathode active material increases. It is assumed that the phenomenon similar to the deterioration by continuous charging discussed above is taking place.
For solving the above problems, it has been contemplated to stabilize the structure of the cathode active material. As means for stabilizing the structure, it is known to mix or substitute a cathode active material lithium cobalt oxide with elements other than cobalt (Co).
For example, Patent Publication 1 proposes to substitute part of Co, which constitutes the crystal structure of a lithium-containing composite oxide, which is a cathode active material, with Na or K for stabilizing the crystal structure.
Patent Publication 2 proposes a cathode active material LiNiO2 containing Co, Al, Mg, and at least one element selected from K, Na, Rb, and Cs, so that these elements are present in the Li layer and exhibit so-called the pillar effect during charging wherein Li is eliminated, to effectively inhibit disintegration of the Li layer.
Patent Publication 1: JP 2004-265863 A
Patent Publication 2: JP 2005-116470 A